The present invention relates to apparatus and to a method for carrying out high temperature treatment of lignocellulosic material, such as wood.
High temperature treatment of lignocellulosic material;, such as wood, makes it possible to reduce their moisture content and improve their stability characteristics.
Various methods and apparatus for carrying out high temperature treatment of lignocellulosic materials are known. FR-A-2,720,969 discloses such a method and a cell for carrying it out. This document discloses drying of the materials, followed by heating in a closed circuit during which the gases released by the material are employed as a fuel, and finally, cooling by injection of water. The closed-circuit heating step disclosed in this document does not make it possible to ensure residual humidity, remaining after the drying step, is completely eliminated. Additionally, the use of the gases released by the material as a fuel involves control of the treatment plant which is difficult to achieve in practice. Finally, injecting water for cooling leads to the material treated splitting or breaking up. The cell disclosed in that document for carrying out the method has corresponding disadvantages, and in practice, it is difficult or even impossible to carry out material treatment inside it. In particular, it is difficult, with this apparatus, to ensure that the gases released are subject to combustion, as proposed in the method, and it is also difficult and dangerous to carry out heating in a closed circuit.
The invention discloses a method and apparatus making it possible to overcome these disadvantages. It provides simple, effective, high temperature treatment, preserving the mechanical properties of the material, and is easy to carry out in practice. The apparatus of the invention has a simple and robust structure, and makes it possible to provide effective treatment without the need for complicated adjustments. The invention notably allows automatic control of treatment.
More precisely, the invention provides apparatus for high temperature treatment of lignocellulosic material, comprising a treatment chamber for the material, at least one combustion chamber with at least one burner operating in a reducing atmosphere, mixing means providing a circulation of treatment chamber gases whereby a portion of the gases originating from the treatment chamber circulate through the combustion chamber.
The apparatus can comprise an induction channel for gases in the treatment chamber, linked to an induction side of said mixing means, a delivery channel for gases to the treatment chamber, linked to a delivery side of the mixing means, a branch on said induction channel, said branch on the induction channel being linked to the combustion chamber, and a branch on the delivery channel, said branch on the delivery channel being linked to the combustion chamber.
In this case, the branch on the induction channel or delivery channel can terminate in the combustion chamber close to the burner.
The apparatus can further comprise at least one extraction chimney on the circulation path of the treatment chamber gases, downstream of said mixing means.
The apparatus can further comprise means for introducing water into the combustion chamber, close to said burner, and can further comprise temperature sensors for measuring a temperature externally of said material and a temperature within said material, and means for regulating one or more burners, whereby a substantially constant difference xcex94 is maintained between a temperature external of said material and a temperature within said material during treatment before cooling, said temperature difference being preferably dependent upon the cross-section of the material to be treated.
The temperature sensors advantageously comprise at least one sensor close to a wall of said treatment chamber for measuring the temperature externally of said material, and at least one mobile sensor arranged to be disposed inside a charge of material, for measuring the temperature within said material.
A method for high temperature treatment of lignocellulosic material is also disclosed, comprising the steps of
pre-heating and drying material at a temperature close to the vaporization temperature of water;
heating said material in a reducing atmosphere;
cooling said material by injecting water vapor.
The pre-heating and drying step is preferably continued until practically all the free water contained in said material has evaporated, preferably until the humidity of gases around said material is less than 12%.
The step of heating said material in a reducing atmosphere is preferably carried out until a temperature comprised between 200 and 240xc2x0 C., and particularly between 220 and 230xc2x0 C., is reached.
The step of heating said material in a reducing atmosphere is advantageously carried out while causing gases released by said material to circulate through a combustion chamber with at least one burner, in a reducing atmosphere.
The step of heating said material in a reducing atmosphere is preferably followed by a step in which said material is maintained at a temperature close to the temperature reached during said heating step.
The step of cooling said material by injecting water vapor can be followed by a cooling step, by circulation of air.
Treatment is advantageously carried out by maintaining a substantially constant difference between a temperature external of said material and a temperature within said material during the steps of pre-heating, drying, and heating, said difference being preferably dependent upon the cross-section of the material to be treated.
Other advantages and characteristics of the invention will become more clear from the description which follows of one embodiment of the invention, provided by way of example and with reference to the attached drawings.